Multiple development method and apparatus for electrophotographic copiers

ABSTRACT

Electrophotographic copying apparatus having a multiple development capability for more completely developing and, hence, improving the quality of images which contain extended solid areas. This multiple development capability is attained by designing the development station in such a manner that the latent electrostatic image to be developed and the developer mixture will move through the development station at a substantially identical velocity as distinguished from conventional systems in which the developer passes through the development station much faster than the image. By matching the velocities of the image and the developer, degradation of the developed, or partially developed, image by the rubbing or sweeping of the carrier beads across the image will be substantially eliminated, and, accordingly, multiple development becomes possible. The substantial matching of velocities is preferably accomplished by significantly reducing the width of the gap between the latent image bearing medium and the development plate through which the developer falls. This increases the density of developer in the gap and the degree of developer agitation and, hence, causes it to fall much slower while still ensuring an adequate supply for effective development of all areas of the image. With a system designed in this way, copies having a quality equal or superior to those obtainable with conventional systems can be achieved with a single development step and at speeds comparable to those available in todays high speed machines. When, however, the image to be developed contains large extended solid areas, any needed number of additional development cycles can be selectively carried out to better develop such images and, hence, provide superior quality copies.

[ Nov. 25, 1975 1 MULTIPLE DEVELOPMENT METHOD AND APPARATUS FOR whichcontain extended solid areas. This multiple development capability isattained by designing the de- ELECTROPHQTOGRAPHIC COPIERS velopmentstation in such a manner that the latent [75] Inventors: Robert A.Moore, Amherst, NH;

Daniel J. Sullivan, Hyde Park, Mass.

[73] Assignee: Itek Corporation, Lexington, Mass.

[22] Filed:

June 24, 1974 image. By matching the velocities of the image and thedeveloper, degradation of the developed, or partially 21 Appl. No.:482,040

developed, image by the rubbing or sweeping of the carrier beads acrossthe image will be substantially eliminated, and

, accordingly, multiple development 76 H m5 001 no ;3 50 G 1. 1. m m 8n1" m 1! 0 Sn um 11. 21 55 ll becomes possible. The substantial matchingof veloci- [58] Field of Search 118/7, DIG. 24, 636, 637;

ties is preferably accomplished by significantly reducing the width ofthe gap between the latent image bearing medium and the developmentplate through which the developer falls. This increases the density ofReferences Cited UNITED STATES PATENTS developer in the gap and thedegree of developer agi- 118/637 tation and, hence, causes it to fallmuch slower while 118/6 still ensuring an adequate supply for effectivedevelop- H3/637 ment of all areas of the image. With a system designedin this way copies having a quality equal or Superior to thoseobtainable with conventional systems can be 3,592,642 3,736,055 Davidgeet al..

3,754,531 Donalies...........

Primary ExaminerW. C. Reynolds achieved with a single development stepand at speeds comparable to those available in todays high speedmachines. When, however, the image to be developed AssistantExaminer-Douglas Salser Attorney, Agent, or Firm-Homer 0. Blair; RobertL. Nathans; Gerald I-l. Glanzman contains large extended solid areas,any needed number of additional development cycles can be selec-ABSTRACT Electrophotographic copying apparatus having a multivelycarried out to better develop such images and, hence, provide superiorquality copies.

tiple development capability for more completely developing and, hence,improving the quality of images 13 Claims, 1 Drawing Figure 051 51OPME/VT CYCLE CONTROL C/RCU/TZ] U.S. Patent Nov. 25, 1975 momDOmMULTIPLE DEVELOPMENT METHOD AND APPARATUS FOR ELECTROPHOTOGRAPHICCOPIERS BACKGROUND OF THE INVENTION 1. Field of the Invention Thepresent invention relates generally to the field of electrophotographiccopiers, and, more particularly, to an electrophotographic copier havinga multiple development capability for improving the quality of imagescontaining extended solid and half-tone areas.

2. Description of the Prior Art In the typical cascade developmentstation of an electrophotographic copier, the latent electrostatic imageto be developed and a developer mixture are brought together such thattoner powder can be attracted to and developed on appropriate areas ofthe latent image. In such systems, to produce high quality copies it isessential that the latent image to be developed, an adequate supply ofdeveloper, and a sufficient degree of developer agitation be maintainedin the development station of a sufficient period of time to ensure thatsubstantially complete development of the entire image can take place.

Although many systems presently available are able to accomplish thisgoal in a generally satisfactory manner when the document to be copiedcontains ordinary line copy or the like, they are not fully acceptablewhen the image to be developed contains large solid or halftone areassuch as pictures, graphs or the like. In such situations, the latentimage must receive especially large amounts of toner to more fullyneutralize the charges on the image and to produce a good quality highdensity black image that approaches the quality obtainable byconventional printing techniques, and most commercial electrostaticsystems are simply not able to accomplish this in an effective manner,and, as a result, generally provide copies in which solid areas are ofan inferior quality.

Most of the suggestions that have been made to solve this problem entailthe act of keeping the latent image to be developed in the developmentstation for a longer period of time than normal such that larger amountsof toner are able to be deposited on the image. For example, onesuggested solution is to increase the size of the development stationand, hence, increase the length of time that the latent image takes inpassing through it. This, however, is not a very good solution because,for one thing, it tends to make the entire copying system larger,bulkier, and more expensive.

Another approach is to leave the size of the developing station alone,but instead cause the latent image to move through it at a slower speed.This is also unsatisfactory because it requires that other operations ofthe machine be accelerated or adjusted to maintain high overalloperating speed and this makes the machine more complex and lessreliable.

In general, there is no system presently available in the marketplacewhich can provide high quality copies of those originals which containlarge extended solid areas in a satisfactory manner without sacrificingmachine compactness and/or efficiency.

SUMMARY OF A PREFERRED EMBODIMENT OF THE INVENTION In accordance with apresently preferred embodiment of the invention, the above inadequacieshave been significantly reduced by providing an electrophotographiccopying system having a multiple development capability which may beutilized in a selective manner to more fully develop those latent images5 which contain extended solid areas. More particularly,

the present invention permits the latent image to be directed throughthe development station a plurality of times as needed or desired toensure that high density black images of pictures, graphs or the likecan be produced.

This multiple development capability is attained by the provision of adevelopment station of novel construction which avoids any significantimage degradation that would normally be expected to occur as a resultof the image passing through the development station more than one time.In the typical system, multiple development would actually tend todamage the developed (or partially developed) image more than enhanceit, because as the image is passed through the development station for asecond or subsequent time, there would be a substantial tendency for thecarrier beads to physically rub against and noticeably damage backgroundareas by producing a high density of black spots thereon, and, also, tocause edges of the image to become less well defined. This would happenbecause in the conventional system, the carrier beads upon which thetoner particles are electrostatically bound are passed through thedevelopment station at a speed substantially in excess of the speed ofthe moving latent image to ensure that adequate amounts of toner will besupplied to all areas of the image and it is this velocity differentialwhich creates the rubbing or sweeping activity.

In the present invention, however, the developing station has beendesigned such that the developer and the latent electrostatic image canbe moved through it at substantially identical speeds such that therewill not be any significant scrubbing or wiping action between them, andbecause there is no significant scrubbing, there will be no significantimage degradation during second and subsequent development operations.

Reduction in the rate of travel of the developer relative to the latentimage can be accomplished in several ways while still ensuring that anadequate supply of toner will be available to all areas of the image andwithout sacrificing machine speed for a single development cycle. In thepresently most preferred embodiment, for example, the spacing or gapbetween the drum that carries the latent electrostatic image and thedevelopment plate or electrode is reduced significantly. This reducedspacing results in a higher particle density and a higher frequency ofdeveloper collisions in the gap and, hence, in a slower movement of thedeveloper through the gap. Also, this increased density will ensure thata large amount of developer be present adjacent each area of the imageto be developed to ensure full development. In addition, the developmentelectrode is preferably designed with a knurled surface in the mannerdisclosed in copending U.S. Patent Application Ser. No. 429,616 filed onJan. 2, 1974 to James E. Genthe and William R. Buchan and entitledCASCADE DE- VELOPMENT STATION HAVING A ROUGHENED DEVELOPMENT PLATE FORENHANCING DE- VELOPER MIXTURE TURBULENCE, now abandoned in favor ofcontinuation-in-part application Ser. No. 547,851, filed on Feb. 6,I975. Such an electrode design permits a better and more uniformspreading of the developer to be effected to further ensure thatsufficient toner will be available to all areas of the image.

In general, the system provided permits multiple development to beselectively utilized in those situations when needed or desired tocompletely develop an image such as when the original contains extendedsolid areas. When multiple development is not needed, such as when theoriginal contains only line copy or when improved image quality is notimportant, only a single development need be performed without any lossin copy speed. Yet further features of the invention will be set outhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS The FIGURE schematically illustratesthe development station of an electrophotographic copier in accordancewith a presently preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The FIGUREillustrates, in somewhat schematic form, the development area of anelectrophotographic copier. Other conventional portions of the copiersuch as the imaging station, the corona charging station, the exposurestation and the drum cleaning station have not been shown in theinterest of clarity and because they do not form a part of the presentinvention.

In the FIGURE, reference number 1 identifies the photoconductive drumupon the surface of which a latent electrostatic image has been formedin the conventional manner. Reference number 2 identifies the developersupply hopper while reference numbeer 3 refers to the development plateor electrode which is spaced from the surface of drum 1 to define a gapor space 4 therebetween as understood by those skilled in the art.

In operation of the system, developer is dispensed from developer hopper2 and cascades downwardly through the gap 4 such that toner will beattracted to the latent electrostatic image on the surface of drum 1being carried through the development station in the direction indicatedby arrow 5. The developer thereafter falls to the bottom of the machinewhere it is usually replenished with fresh toner and is picked up byconveyer 6 controlled by a motor 10 which carries it back to the top ofthe machine to be recirculated therethrough.

After the latent image has been suitably developed on the drum 1, it isthen transferred to a suitable transfer belt 8 of the type described incopending US Patent Application Ser. No. 403,696 by William R. Buchanand Robert A. Moore, filed on Oct. 4, 1973, and entitledELECTROPHOTOGRAPHIC TONER TRANSFER AND FUSING APPARATUS AND METHOD, orto some other medium in a manner rec ognized by those skilled in theart. In particular, belt 8 is adapted to be carried across the drum inthe direction indicated by arrow 7 by a suitable roller system 9, 11 andcan be selectively brought into or out of contact with the drum by meansof a spring roller 12 or the like as will be explained in greater detailhereinafter. Belt 8 and its associated structure does not form part ofthe present invention, and is illustrated schematically only to helpexplain the invention. It should be clearly understood, however, thatthe present invention could also be employed in those systems wherein animage is transferred directly from the photoconductive drum to paper orto other mediums using electrostatic transfer.

Development plate 3 functions as a metallic electrode and is thuselectrically connected to a conventional bias source 13 as is wellrecognized by those skilled in the art. Preferably also, developmentelectrode 3 is provided with a knurled surface of the type described inthe earlier-mentioned copending US. Patent Application Ser. No. 429,616which assists in distributing the developer more uniformly over theentire image to be developed.

An important feature of the invention is that the gap 4 formed betweenthe surface of drum 1 and plate 3 is designed to be substantiallynarrower than those employed in conventional systems. Specifically, inthe presently constructed machine, gap 4 has a spacing of approximately175 mils in region 16, a spacing of about mils in region 17, a spacingof about 85 mils in re gion 18 and a spacing of about mils in region 19.This variable spacing has been provided to ensure a rel atively smoothflow of developer into and out of gap 4 without creating excessivepile-ups while maintaining the very narrow spacing desired in the longercentral region of the gap. Other spacings, both variable and uniform mayalso be employed if desired, however, and, as a general rule, a spacingof from about 50 mils to about 300 mils with central areas of the gappreferably being less than about lOO mils would satisfy the requirementsof the present invention and permit multiple development.

The toner mixture itself is of conventional type consisting of carrierbeads having toner powder adhered thereto and having an average diameterof about 15 mils. Obviously, for different sized beads, somewhatdifferent gap widths might be employed.

With the development station constructed as described above, theelectrophotographic copier will, in only a single development cycle,produce copies of a quality equal to or better than those obtainable inpresent systems, At the same time, it will also permit images to bedeveloped a plurality of times so that those images which contain largeor extended solid areas can be more completely developed than ispossible with conventional systems. The reason why the present invention is able to do this can perhaps be better understood by brieflydescribing the operation of conventional copiers.

Basically, in conventional development stations the width of gap 4 iskept at a relatively large size, for example, 250 mils. As a result, thecarrier beads tend to fall through the gap quite freely at a velocitywhich is significantly higher than the velocity of the drum surface(which is preferably about lO inches/sec. for ap proximately 5 inchdiameter drum) e.g. twice as fast or more. This high developer velocityis provided intentionally and generally believed to be necessary so asto ensure that an adequate supply of fresh toner will be supplied to allareas of the latent image as it is passing through the developmentstation. Because of this velocity difference between the developer andthe drum surface, however, there will be a substantial amount ofphysical contact between the carrier beads and the latent image on thedrum in the form of a tangential rubbing or wiping action of the beadsagainst the image. Because of this rubbing action, second or subsequentdevelopment cycles would cause unacceptably high background density, andhence make multiple development procedures unacceptable. Furthermore,because gap 4 is relatively wide, conventional systems requirerelatively large amounts of developer and this tends to compound theproblem because with more developer there will be more rubbing. Thus, inthese conventional systems, in order to obtain high densityblack areasof good quality it has been though to be necessary to either sacrificemachine speed or'machine compactness by utilizing one of the techniquesdescribed previously.

These deficiencies in prior art, however, have been overcome in thepresent invention by making the width of gap 4 substantially narrowerthan in the prior art. Because thegap 4 is quite narrow, the developerfalling through it will be squeezed together into a higher collisionfrequency than in the prior'artpFor this reason, they will continuouslybe contacting and interacting with one another and thus be forced tofall at a much lower velocity thanin the prior art and, in fact, willfall at a velocity substantially equal to or only slightly greater thanthat of the surface of the rotating drum. Because the carrier beads andthe drum surface are moving at substantially the same velocity, theamount of scrubbing or wiping between them will be reduced to a minimumand there will not be any significant image degradation during second orsubsequent development cycles and, hence, multiple development becomespossible. In fact, it has actually been found that the quality of evenbackground portions is somewhat improved by the multiple developmentprocedure. This is believed to be because in later development cycles,any background toner will tend to be scavenged by the depleted orpartially depleted carrier beads and this is noticeable in improving thequality of background areas of the image.

It should also be emphasized that notwithstanding the fact that thedeveloper and the drum surface move through the development station atsubstantially the same velocity, there is no reduction in the degree ofdevelopment that takes place with only a single development cycle'overthat found in the prior art. This is because in because in the presentinvention, although the developer is moving slower than in the priorart, it is more densely packed in the gap and, hence, there is still anadequate supply of toner available to develop the image. Also, becausethe developer is moving slower, there is more time for thelatentelectrostatic image to attract powder to it than in the prior art.Finally, by also providing the knurled development electrode asdescribed in the above-referred to US. Patent Application Ser. No.'429,616, there is substantial assurance that all areas of the image willreceive an adequate supply of toner. For these reasons, when the imageto be copied contains only line copy or when time is of the essence, asingle development cycle at speeds at least comparable to those found inthe fastest present-day machines will produce a copy as good as orbetter than those presently obtainable, and it is only when the originalcontains large solid areas, that multiple development need be used inorder to significantly improve the quality of such images.

The number of development cycles desired without image transfer to belt8 can be selected as desired by means of control 21 mounted in themachine console. If the image contains primarily line copy, a singledevelopment is generally all that is needed. If, however, there areextended dark areas on the image, additional cycles, as required, can beprovided merely by turning the dial on control 21. Although essentiallyas many development cycles as desired can be carried out, (the presentinvention has actually been practiced with more than 10 cycles withoutimage degradation) the image is usually fully developed after aboutthree cycles and this is all that shouldnormally be necessary. After thedesired inumber .of development cycles has been carried out, control 21willautomatically actuate roller 12 and belt driving rollers 9 11 tomove the belt 8 into contact with the drum for transfer of the imagethereto.

Thus, with the present invention a development station forelectrophotographic copiers has been provided which can selectively becontrolled to develop an image as many times as desired or as needed forthe particular type of image being copied. Thesystem is highly efficientbecause all that is required-is that a single control be adjusted. Ifthere are no extended dark areas on the image, a single development isall that is required and the system will work as fast and as efficientlyas any other. If there are extended dark areas on the image, it will, ofcourse, take longer to produce copies but this will be more thancompensated for by the better quality image.

It should be understood that the above-described em.- bodiment is apresently preferred embodiment only, For example, the multipledevelopment capability described above can also be used to morecompletely develop images on zinc oxide sheets or the like to be used inplate making operations. In such applications, the developed image wouldnot be transferred to a belt or paper as described above, but, instead,would be used directly for printing. As used herein, the termelectrophotographic copier is also intended to include such anapplication. Because many additions, alterations and omissions may bemade from the invention without departing from the scope thereof, itshould be recognized that it should be limited only insofar as requiredby the scope of the following claims.

What is claimed is: I

1. In an electrophotographic copying apparatus including:

a first support medium carrying a latent electrostatic image to bedeveloped thereon; a second support medium for receiving the developedimage from said first support medium;

transfer means for transferring said developed image from said firstsupport medium to said second support medium; and,

developing means for developing said latent electrostatic image on saidfirst support medium prior to the transfer thereof to said secondsupport medium by said transfer means, the improvement comprismg:

wherein said developing means includes control means for selectivelycontrolling the number of times that said latent electrostatic image isdeveloped prior to the transfer thereof from said first support mediumto said second support medium by said transfer means.

2. Apparatus as recited in claim 1 wherein said control means furtherincludes selection means for selecting the number of times that saidlatent electrostatic image is to be developed prior to the transferthereof from said first support medium to said second support medium,said number of times being from one to at least three.

3. Apparatus as recited in claim 1 wherein said developing means furtherincludes:

a. a development station;

b. drive means coupled to said first support medium for passing saidlatent electrostatic image through said development station;

c. means for supplying developer to said development station as saidlatent electrostatic image is passed therethrough for developing saidlatent electrostatic image; and,

d. wherein said control means includes means coupled to said drive meansfor passing said latent electrostatic image through said developmentstation said selected number of times prior to the transfer thereof fromsaid first support medium to said second support medium.

4. Apparatus as recited in claim 3 wherein said devel oper supplyingmeans includes means for passing said developer through said developmentstation at a velocity substantially equal to the velocity that saidlatent electrostatic image is passed through said development station bysaid drive means.

5. Apparatus as recited in claim 4 wherein said development stationfurther includes a development plate spaced from said first supportmedium to define a narrow gap therebetween, at least a portion of saidgap having a width of less than about 100 mils. and wherein said meansfor passing said developer through said development station comprisesmeans for dropping said developer through said gap, the narrow width ofsaid gap causing said developer to fall at a velocity substantiallyequal to the velocity of said latent electrostatic image.

6. Apparatus as recited in claim 5 wherein said first support mediumcomprises a rotatable drum having a surface upon which said latentelectrostatic image is formed.

7. Apparatus as recited in claim 6 wherein said development plateincludes a knurled surface.

8. Apparatus as recited in claim 1 wherein said transfer means includesmeans coupled to said control means for bringing said second supportmedium into contact with said first support medium after said selectednumber of developing steps for transferring said developed image fromsaid first medium to said second medium.

9. Electrophotographic copying apparatus comprismg:

a. a first support medium carrying a latent electrostatic image to bedeveloped;

b. a second support medium for receiving the developed image from saidfirst support medium;

c. means for transferring said developed image from said first supportmedium to said second support medium;

d. developing means for developing said latent electrostatic image onsaid first support medium prior to the transfer thereof to said secondsupport medium by said transfer means; and,

e. control means for controlling the number of times that said latentelectrostatic image is developed prior to the transfer thereof from saidfirst support medium to said second support medium by said transfermeans.

10. Apparatus as recited in claim 9 wherein said control means includesselection means for selecting the number of times that said latentelectrostatic image is to be developed prior to the transfer thereoffrom said first support medium to said second support medium, saidnumber of times being from one to at least two.

11. Apparatus as recited in claim 9 wherein said developing meansfurther includes:

a. a development station;

b. drive means coupled to said support medium for passing said latentelectrostatic image through said development station;

0. means for supplying developer to said development station as saidlatent electrostatic image is passed therethrough for developing saidlatent electrostatic image; and,

d. wherein said control means includes means coupled to said drive meansfor passing said latent electrostatic image through said developmentstation a selected number of times prior to the transfer thereof fromsaid first support medium to said second support medium.

12. Apparatus as recited in claim 11 wherein said developer supplyingmeans includes means for passing said developer through said developmentstation at a velocity substantially equal to the velocity that saidlatent electrostatic image is passed through said development station bysaid drive means.

13. Apparatus as recited in claim 12 wherein said development stationfurther includes a development plate spaced from said support medium todefine a narrow gap therebetween, at least a portion of said gap havinga width of less than about mils. and wherein said means for passing saiddeveloper through said development station comprises means for droppingsaid developer through said gap by gravity, the narrow width of said gapcausing said developer to fall at velocity substantially equal to thevelocity of said latent electro-

1. In an electrophotographic copying apparatus including: a firstsupport medium carrying a latent electrostatic image to be developedthereon; a second support medium for receiving the developed image fromsaid first support medium; transfer means for transferring saiddeveloped image from said first support medium to said second supportmedium; and, developing means for developing said latent electrostaticimage on said first support medium prior to the transfer thereof to saidsecond support medium by said transfer means, the improvementcomprising: wherein said developing means includes control means forselectively controlling the number of times that said latentelectrostatic image is developed prior to the transfer thereof from saidfirst support medium to said second support medium by said transfermeans.
 2. Apparatus as recited in claim 1 wherein said control meansfurther includes selection means for selecting the number of times thatsaid latent electrostatic image is to be developed prior to the transferthereof from said first support medium to said second support medium,said number of times being from one to at least three.
 3. Apparatus asrecited in claim 1 wherein said developing means further includes: a. adevelopment station; b. drive means coupled to said first support mediumfor passing said latent electrostatic image through said developmentstation; c. means for supplying developer to said development station assaid latent electrostatic image is passed therethrough for developingsaid latent electrostatic image; and, d. wherein said control meansincludes means coupled to said drive means for passing said latentelectrostatic image through said development station said selectednumber of times prior to the transfer thereof from said first supportmedium to said second support medium.
 4. Apparatus as recited in claim 3wherein said developer supplying means includes means for passing saiddeveloper through said development station at a velocity substantiallyequal to the velocity that said latent electrostatic image is passedthrough said development station by said drive means.
 5. Apparatus asrecited in claim 4 wherein said development station further includes adevelopment plate spaced from said first support medium to define anarrow gap therebetween, at least a portion of said gap having a widthof less than about 100 mils. and wherein said means for passing saiddeveloper through said development station compriseS means for droppingsaid developer through said gap, the narrow width of said gap causingsaid developer to fall at a velocity substantially equal to the velocityof said latent electrostatic image.
 6. Apparatus as recited in claim 5wherein said first support medium comprises a rotatable drum having asurface upon which said latent electrostatic image is formed. 7.Apparatus as recited in claim 6 wherein said development plate includesa knurled surface.
 8. Apparatus as recited in claim 1 wherein saidtransfer means includes means coupled to said control means for bringingsaid second support medium into contact with said first support mediumafter said selected number of developing steps for transferring saiddeveloped image from said first medium to said second medium. 9.Electrophotographic copying apparatus comprising: a. a first supportmedium carrying a latent electrostatic image to be developed; b. asecond support medium for receiving the developed image from said firstsupport medium; c. means for transferring said developed image from saidfirst support medium to said second support medium; d. developing meansfor developing said latent electrostatic image on said first supportmedium prior to the transfer thereof to said second support medium bysaid transfer means; and, e. control means for controlling the number oftimes that said latent electrostatic image is developed prior to thetransfer thereof from said first support medium to said second supportmedium by said transfer means.
 10. Apparatus as recited in claim 9wherein said control means includes selection means for selecting thenumber of times that said latent electrostatic image is to be developedprior to the transfer thereof from said first support medium to saidsecond support medium, said number of times being from one to at leasttwo.
 11. Apparatus as recited in claim 9 wherein said developing meansfurther includes: a. a development station; b. drive means coupled tosaid support medium for passing said latent electrostatic image throughsaid development station; c. means for supplying developer to saiddevelopment station as said latent electrostatic image is passedtherethrough for developing said latent electrostatic image; and, d.wherein said control means includes means coupled to said drive meansfor passing said latent electrostatic image through said developmentstation a selected number of times prior to the transfer thereof fromsaid first support medium to said second support medium.
 12. Apparatusas recited in claim 11 wherein said developer supplying means includesmeans for passing said developer through said development station at avelocity substantially equal to the velocity that said latentelectrostatic image is passed through said development station by saiddrive means.
 13. Apparatus as recited in claim 12 wherein saiddevelopment station further includes a development plate spaced fromsaid support medium to define a narrow gap therebetween, at least aportion of said gap having a width of less than about 100 mils. andwherein said means for passing said developer through said developmentstation comprises means for dropping said developer through said gap bygravity, the narrow width of said gap causing said developer to fall atvelocity substantially equal to the velocity of said latentelectrostatic image.